A large number of different materials are used in automobile parts, such as metals, glass, rubber and plastics. Of those, thermoplastic plastics contribute to reduce weight of automobiles, are subject to less restrictions in designing automobile parts, and allow high productivity of the parts made thereof and easy reduction in production costs. Because of these advantages, use of plastics for automobile production has been steadily increasing in recent years.
For the automobile parts for which the required properties are relatively mild, general-purpose plastics such as polyethylene and polypropylene have been used since relatively early times. With the increasing trend of the recent years to substitute with plastics even for the parts which are used under severe environmental conditions such as high temperature and high humidity of an engine room, more and more engineering plastics such as polyamide, polyacetal, polybutylene terephthalate (hereinafter abbreviated as PBT), etc., excelling in mechanical strength and heat resistance are used.
In particular, materials to be used for the parts that come into direct contact with fuel, various oils and/or combustion gas are required to have properties such as high resistance to chemicals, oil, gasoline, hot water, alkalis and acids, etc.
Each of known engineering plastics has both sides of advantageous characteristics and defective characteristics. For example, polyamide excels in mechanical properties, heat resistance, oil resistance, gasoline resistance and moldability, but due to its high water absorption, it exhibits appreciable dimensional change or drop in mechanical strength upon absorbing water. It is also insufficient in chemical resistance, as it tends to develop stress cracks in the presence of metal salts such as calcium chloride.
PBT excels in mechanical properties, heat resistance, resistance to thermal aging, electrical characteristics, chemical resistance and moldability. However, it exhibits poor resistance to hot water, due to which defect some parts made thereof are low in reliability and unfit for practical use.
Those characteristics of engineering plastics are attributable to primary structures of respective polymers and the properties of specific groups contained therein, such as amide groups or ester groups. Therefore, attempts to improve the defects by use of additives or polymer blends cannot offer fundamental solutions.
Substitution of a metal case with a polyamide case has been also advancing for a case of engine canister filled with activated carbon, which has a function to absorb vaporized gasoline generating from a gasoline tank or engine. As already stated, however, polyamide tends to form stress cracks in the presence of a metal salt, and besides, it shows high "gasohol" permeability. Therefore, improvements on these points are in demand.
Here the "gasohol" is a mixture of gasoline and methanol, which serves as a fuel for methanol engines.